PROJECT SUMMARY The proposed study links with and leverages an ongoing scale up study of a cognitive behavioral therapy (CBT) based intervention, the Youth Readiness Intervention (YRI), among youth facing adversity in Sierra Leone that is currently being implemented in partnership with the Government of Sierra Leone (GoSL) and the Gesellschaft fr Internationale Zusammenarbeit (GIZ). The YRI has demonstrated feasibility and effectiveness for improving emotion regulation and daily functioning in youth. This study aims to harness the mechanisms of natural diffusion of CBT techniques learned among peer groups and spillover phenomenon in reduction of burden to cohabitating caregivers. Preliminary studies of the YRI in Sierra Leone indicate both mechanisms occur. Researchers from the Boston College School of Social Work will examine the potential of the YRI to reach a larger segment of the population by examining the extent to which indirect effects result in measureable incremental health benefits (symptoms and functioning) among nonparticipants. Study aims are to investigate (1) mechanism of diffusion?the untargeted and unplanned spread of new practices among social network members?of YRI components and mental health benefits experienced by nonparticipant peers who learn YRI practices from YRI participants; (2) indirect mental health benefits experienced by nonparticipants among cohabitating caregivers of YRI participants (spillover effects); and (c) incremental health costs and benefits among YRI participants' caregivers and peers through cost-effectiveness and return on investment analysis. Study design includes a control group (current YRI participants and YRI nonparticipants). YRI participants and control participants who consent to the current study will nominate 3 peers in their social networks aged 18-24 and will also identify their primary cohabitating caregiver. Following consent, peers and caregivers will be assessed at baseline and 6-month follow-up on mental health, emotion regulation, daily functioning, and sense of burden (caregivers only). Study outcomes also incorporate common indicators for implementation science, including measures of project context, evaluation, and scale-up, to enhance knowledge exchange across global research networks. Assessing implementation research outcomes, including penetration of YRI effects and cost-effectiveness of the YRI as distinct outcomes will provide key information about the success of YRI implementation, thereby supporting decisions about whether to increase scale up efforts in Sub-Saharan Africa and other LMICs. Findings on diffusion of the YRI will inform the feasibility of peer-led interventions and the development of additional educational materials to amplify YRI components identified as most transferrable. Findings on spillover effects will inform the potential reach and penetration of evidence-based practices in LMICs. Lessons learned will inform the GoSL, GIZ, and development actors on investing in evidence-based mental health interventions in low-resource settings.